A well-known and often used method fo speed control for an A.C. induction motor is the "constant volts per hertz" technique. This method is based on the inherent speed regulation capability of the A.C. induction motor, which is that its synchronous, no-load speed is proportional to the excitation frequency according to the relationship NRPM =(120.times.frequency) number of poles. Thus, a four pole A.C. induction motor has a synchronous speed of 1800 RPM at 60 Hz. of excitation. The "constant volts per hertz" means that if the motor excitation is lowered to 30 Hz., the voltage applied to the motor must be lowered to half of its value at 60 Hz., and if the motor excitation is raised to 120 Hz. the voltage applied to the motor must be doubled to achieve the same output current. But the speed regulation of the "constant volts per hertz" method is relatively inaccurate because the torque producing mechanism of the motor requires slip or a reduction in motor speed from its synchronous speed. The lowest slip induction motors available are rated at 2% slip, and therefore, 2% regulation is the best that can be achieved with the "constant volts per hertz" method.
In the past, when more accurate speed regulation was required, a tachometer had to be employed along with means for varying the frequency of excitation to achieve a predetermined output of the tachometer. But a tachometer is relatively expensive, takes up space, and places speed limitations on the motor that are undesirable in high speed applications. Accordingly, the principal object of this invention is to provide motor control circuit which has highly accurate speed regulation but does not require a tachometer. Other objects and advantages of the invention will be apparent from the detailed description herein.